System and method for high resistance ground fault detection and protection in power distribution systems

ABSTRACT

A system and method for detecting high resistance ground faults in a power distribution system is disclosed. A fault detection and protection system is provided that includes a plurality of current sensors to measure current on the three phase output of the converter-inverter arrangement of the power distribution system and a controller configured to measure the three phase current on the three phase output, extract a fundamental current component for each phase of the three phase output, extract a third harmonic component for each phase of the three phase output, compare the fundamental current component and the third harmonic component extracted from each phase to a first threshold and a second threshold, respectively, and detect a ground fault on a phase of the three phase output based on the comparisons of the fundamental current component and the third harmonic component to the first and second thresholds.

BACKGROUND OF THE INVENTION

The present invention relates generally to power distribution systemsand, more particularly, to a system and method for detecting highresistance ground faults in a power distribution system and protectingthe power distribution system from such ground faults upon detectionthereof.

A typical power distribution system includes a converter, an inverterand a mechanical load such as a motor. The converter is typically linkedto a three phase source that provides three phase AC power and convertsthe three phase power to DC power across positive and negative DC buses.The DC buses feed the inverter which generates three phase AC power onoutput lines that are provided to the load. The inverter controls thethree phase AC voltages and currents to the load so that the load can bedriven in a desired fashion. Cables connect the power source to theconverter and also connect the inverter to the load.

One common type of three-phase power distribution system is anadjustable speed drive (ASD). ASDs are frequently used in industrialapplications to condition power and otherwise control electric drivenmotors such as those found with pumps, fans, compressors, cranes, papermills, steel mills, rolling mills, elevators, machine tools, and thelike. ASDs typically provide a volts-per-hertz or vector controls andare adept at providing variable speed and/or variable torque control toan electric driven motor, such that ASDs have greatly improved theefficiency and productivity of electric driven motors and applications.

Power distribution systems such as ASDs require protection frominadvertent cable and load (e.g., motor) failures which can lead toundesirable ground faults. The root cause of cable failures is oftencable insulation breakdown and therefore most ground faults occur in thecables between the power source and the converter or between theinverter and the load. When a ground fault occurs, the results can beextremely costly. For instance, ground faults often result in powerinterruptions, equipment failure and damage, uncoordinated systemdecisions with potential for overall plant interruptions, degraded orlost production and overall customer frustration.

Resistance grounding systems are used in industrial electrical powerdistribution facilities to limit phase-to-ground fault currents.Generally speaking, there are two types of resistors used to tie a powerdistribution system's neutral to ground: low resistance and highresistance. High Resistance Ground (HRG) systems limit the fault currentwhen one phase of the system shorts or arcs to ground, but at lowerlevels than low resistance systems. In the event that a ground faultcondition exists, the HRG typically limits the current to 5-10A, thoughmost resistor manufacturers label any resistor that limits the currentto 25A or less as high resistance.

HRG systems are commonly seen in industrial applications where continuedoperation is important to the process, such as in power distributionsystems where any power source downtime has a dramatic economic cost.HRG systems have gained popularity in such applications due to theirability to continue operation in lieu of a single line-ground fault andimproved ability to limit escalation of the single line-ground faultinto a multi-phase event. Additionally, HRG systems function to suppresstransient line to ground over voltages during a ground fault, eliminatearc flash hazards with phase to ground faults, and reduce equipmentdamage at the point of ground fault.

Since ground fault conditions in HRG systems do not draw enough currentto reliably trigger fault current sensors in an associated motor drive,ground fault detections systems must be employed to detect HRG faults.Various such ground fault detection systems and methods have previouslybeen implemented to locate ground faults. For example, in US PublicationNo. 2009/0296289, detection of a ground fault in the HRG system isaccomplished by injecting a common mode voltage into the three phasesystem and measuring the system response, with the sensed outputvoltages then being filtered to determine the HRG fault occurrence. Inanother example, and as set forth in US Publication No. 2009/0080127,detection of a ground fault in the HRG system is accomplished bymeasuring the DC bus voltage in the HRG system. However, while suchsystems function to detect a ground fault in the HRG system, the methodsemployed in those systems are either computationally cumbersome orintrusive to the system. Additionally, existing ground fault detectionsystems and methods fail to locate the ground fault in the HRG system(i.e., identify where and which phase the fault occurs on). As such,challenges remain in HRG systems with respect to identifying HRG faultsin a cost effective manner and locating the HRG fault in the system.

It would therefore be desirable to provide a system and method thatprovides a computationally efficient approach to detect an HRG fault ina three-phase power distribution system and identify the HRG faultlocation in a particular phase.

BRIEF DESCRIPTION OF THE INVENTION

Embodiments of the present invention provide a system and method fordetecting HRG faults in a power distribution system and identifying thelocation of such ground faults.

In accordance with one aspect of the invention, a power distributionsystem includes a converter-inverter arrangement having an inputconnectable to a three phase AC power source and a three phase outputconnectable to an input terminal of a load, the converter-inverterarrangement configured to control current flow and terminal voltages inthe load. The power distribution system also includes a fault detectionand protection system connected to the converter-inverter arrangement,with the fault detection and protection system including a plurality ofcurrent sensors configured to measure a current on the three phaseoutput of the converter-inverter arrangement and a controller configuredto measure the three phase current on the three phase output of theconverter-inverter arrangement, extract a fundamental current componentfor each phase of the three phase output of the converter-inverterarrangement, extract a third harmonic component for each phase of thethree phase output of the converter-inverter arrangement, compare thefundamental current component and the third harmonic component extractedfrom each phase to a first threshold and a second threshold,respectively, and detect a ground fault on a phase of the three phaseoutput based on the comparisons of the fundamental current component andthe third harmonic component to the first and second thresholds.

In accordance with another aspect of the invention, a method is providedfor detecting a high resistance ground fault in a power distributionsystem that includes an AC motor drive in series between an AC powersource and an AC motor, with the AC motor drive configured to conditiona three phase output to the AC motor. The method includes measuringcurrent on each of a first phase, a second phase, and a third phase ofthe three phase output to the AC motor, extracting a fundamental currentcomponent for each of the first phase, second phase, and third phase,and extracting a third harmonic component for each of the first phase,second phase, and third phase. The method also includes comparing thefundamental current component and the third harmonic component extractedfrom each of the first phase, second phase, and third phase to afundamental component threshold and a third harmonic threshold,respectively and detecting a ground fault on any of the first phase,second phase, and third phase based on the comparisons of thefundamental current component and the third harmonic component on eachphase to the fundamental and third harmonic thresholds. If a groundfault is detected, the method further includes identifying on which ofthe first phase, second phase, or third phase the ground fault ispresent.

In accordance with yet another aspect of the invention, a system fordetecting a ground fault in a high resistance ground (HRG) powerdistribution system includes a plurality of current sensors to measurecurrent on a three phase output of an inverter in the HRG powerdistribution system and a controller configured to measure the threephase current on the three phase output of the inverter, extract afundamental current component for each phase of the three phase output,extract a third harmonic component for each phase of the three phaseoutput, compare the fundamental current component and the third harmoniccomponent extracted from each phase to a fundamental component thresholdand a third harmonic component threshold, respectively, and detect aground fault on a phase of the three phase output based on thecomparisons of the fundamental current component and the third harmoniccomponent on each phase to the fundamental component and third harmoniccomponent thresholds.

Various other features and advantages of the present invention will bemade apparent from the following detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate preferred embodiments presently contemplated forcarrying out the invention.

In the drawings:

FIG. 1 a schematic of an adjustable speed motor drive (ASD) in a highresistance ground (HRG) configuration, according to an embodiment of theinvention.

FIG. 2 is a flowchart illustrating a technique for detection of a HRGground fault in the ASD of FIG. 1, according to an embodiment of theinvention.

FIG. 3 is a graph illustrating HRG fault characteristics in the ASD ofFIG. 1 before and after occurrence of a fault.

FIG. 4 is a graph illustrating HRG fault harmonics characteristics inthe ASD of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The embodiments of the invention set forth herein relate to a system andmethod for detecting HRG faults in a power distribution system andprotecting the power distribution system from such ground faults upondetection thereof.

Embodiments of the invention are directed to power distribution systemsencompassing a plurality of structures and control schemes. According toan exemplary embodiment of the invention, and as shown in FIG. 1, apower distribution system 10 is provided that may be implanted withembodiments of the invention. In the embodiment of FIG. 1, powerdistribution system 10 includes an AC motor drive 11 in the form of anadjustable speed drive (ASD) is shown in FIG. 1. The ASD 11 may bedesigned to receive a three AC power input, rectify the AC input, andperform a DC/AC conversion of the rectified segment into a three-phasealternating voltage of variable frequency and amplitude that is suppliedto a load. In a preferred embodiment, the ASD 11 operates according toan exemplary volts-per-hertz or vector control characteristics. In thisregard, the motor drive provides voltage regulation in steady state, andfast dynamic step load response over a full load range.

In an exemplary embodiment, a power source 12 generates a three-phase ACinput 12 a-12 c is fed to power distribution system 10. According toembodiments of the invention, the power source 12 that generates thethree-phase AC input 12 a-12 c may be in the form of a delta or wyesource transformer, although other arrangements and power sourceconfigurations are also recognized as being able to provide three-phaseAC input 12 a-12 c. The three-phase AC input 12 a-12 c is provided to athree-phase rectifier bridge 14. The input line impedances are equal inall three phases. The rectifier bridge 14 converts the AC power input toa DC power such that a DC link voltage is present between the rectifierbridge 14 and a switch array 16. The link voltage is smoothed by a DClink capacitor bank 18. The switch array 16 is comprised of a series ofinsulated gate bipolar transistor switches 20 (IGBTs) and anti-paralleldiodes 22 that collectively form a PWM (pulse width modulated) inverter24. The PWM inverter 24 synthesizes AC voltage waveforms with a fixed ora variable frequency and amplitude for delivery to a load, such as aninduction motor 26. DC link chokes L1, L2 (indicated in FIG. 1 as 28)are also provided in AC motor drive 11 and are positioned on thepositive and negative rails of the DC link 30. The DC link chokes 28provide energy storage and filtering on the DC link during operation ofAC motor drive 11 and motor 26.

Control of AC motor drive 11 and operation of the inverter 24 is via acontroller 32, which may further be comprised of a plurality ofcontrollers that perform high speed operations such as volts-per-hertzor vector control algorithms, space-vector modulation, DC link voltagedecoupling, and protection, for example. The controller 32 interfaces tothe PWM inverter 24 via gate drive signals and sensing of the DC linkvoltage and pole currents (by way of a voltage sensor 34 for example)such that changes in DC link voltage can be sensed. These voltagechanges can be interpreted as transient load conditions and are used tocontrol switching of the switch array 16 of PWM inverter 24 such thatnear steady-state load conditions are maintained. Additionally,controller 32 functions to identify ground current related faults inpower distribution system 10 and protect the power distribution systemfrom such faults, including protecting the power source transformer 12such as by preventing transformer winding insulation degradation. Inperforming such a fault detection and protection, control systemreceives three-phase output current as input, while outputting faultidentification and protection signals responsive to the inputs, as willbe explained in greater detail below.

According to an exemplary embodiment of the invention, and as shown inFIG. 1, power distribution system 10 is configured as a High ResistanceGround (HRG) system that limits phase-to-ground fault currents. That is,the HRG system provides protection from inadvertent cable and load(e.g., motor) failures that can lead to undesirable ground faults bylimiting the fault current when one phase of the system shorts or arcsto ground. In order to configure power distribution system 10 as an HRGsystem, a resistor 36 (R_(n)) is placed between the three-phase neutralpoint of the power source 12 and ground 38. In a wye connected system,the resistor 36 is placed between the three-phase neutral point of thepower source 12 and ground 38, while in a delta connected system, anartificial neutral is created by using a zig-zag transformer and thenthe resistor 36 is placed between the artificial neutral and ground 38.The resistor 36 is configured to limit the current to 5-10 amps, forexample, in the event that a ground fault condition exists.

Also shown in FIG. 1 is the resistance value in the faulted location inASD 11, identified as a resistor 40 (R_(f)), with such a fault occurringin the form of a short through a wire/cable, breaker, or commutator, forexample. The resistance value of R_(f) can vary based on the particularfault that occurs and can range from zero to a larger value that istypically less than the resistance of resistor 36. The resistance valuein the faulted location in ASD 11 therefore contributes to the overallfault current in power distribution system 10.

As shown in FIG. 1, control system 32 receives three-phase outputcurrent as inputs, such as by way of current sensors 42, 44, 46positioned on the three-phase output of ASD 11. Based on an analysis ofthe three-phase current, control system 32 functions to identify HRGfaults in power distribution system 10 and protect the powerdistribution system from such faults, including protecting againsttransformer winding insulation degradation.

Referring now to FIG. 2, and with continued reference to FIG. 1, anexemplary embodiment of a technique 50 for detection of a HRG fault in apower distribution system, as well as for locating such an HRG fault inthe system, is shown according to an embodiment of the invention. Thetechnique 50 may be implemented by way of a controller associated withan ASD, such as controller 32 connected to ASD 11. As shown in FIG. 2,technique 50 begins at STEP 52 with a measuring of the phase currentfrom each of the three phases of the ASD output, such as by way ofcurrent sensors 42, 44, 46. Upon a measurement of the phase currents ofthe three-phase output, technique 50 continues at STEP 54, where afiltering of the three-phase current is performed in order to accountfor/filter out a high frequency current ripple that is present due toIGBT switching in PWM inverter 24. For example, if a switching frequencyof 5 kHz is used in the PWM inverter, then a low pass filtering can beperformed to remove the 5 kHz and higher frequency current ripplecomponents.

The measured phase current for each phase is therefore defined as:i=I ₁*cos θ+I₃*cos 3 θ  [Eqn. 1],where I₁ is the amplitude of the fundamental current component, I₃ isthe amplitude of the third harmonic component (if it exists), and 0 isthe speed at which the load is commanded to run. According to oneembodiment of the invention, the fundamental component/fundamentalfrequency is 60 Hz and the third harmonic component is 180 Hz, as isstandard in North America, although it is recognized that thefundamental component and the third harmonic component could be 50 Hzand 150 Hz, respectively, as is standard in Europe.

According to an exemplary embodiment, the three-phase output current ofthe ASD 11 is measured periodically over a set period of time, with aplurality of current measurements for each phase being stored incontroller 32 and a current value for each phase being calculated basedon the stored plurality of current measurements for each phase. In anext step of technique 50, the phase current is then analyzed in orderto extract a fundamental current component for each phase of thethree-phase output, as indicated at STEP 56. The fundamental currentcomponent for each phase of the three-phase output is extractedaccording to:

$\begin{matrix}{{\frac{1}{2\pi}{\int_{0}^{2\pi}{\left( {{I_{1}\cos\;\theta} + {I_{3}\cos\; 3{\mathbb{d}\theta}}} \right)*\cos\;\theta*{\mathbb{d}\theta}}}} = \frac{I_{1}}{2}} & \left\lbrack {{Eqn}.\mspace{14mu} 2} \right\rbrack \\{{{\frac{1}{2\pi}{\int_{0}^{2\pi}{\left( {{I_{1}\cos\;\theta} + {I_{3}\cos\; 3{\mathbb{d}\theta}}} \right)*\sin\;\theta*{\mathbb{d}\theta}}}} = 0},} & \left\lbrack {{Eqn}.\mspace{14mu} 3} \right\rbrack\end{matrix}$where I₁ is the amplitude of the fundamental current component, I₃ isthe amplitude of the third harmonic component, and 0 is the speed atwhich the load is commanded to run. In extracting the fundamentalcurrent component for each phase of the three-phase output, both [Eqn.2] and [Eqn. 3] are performed in order to run a self-check, with theoutput of [Eqn. 3] being zero when the calculations are performedproperly. It is noted that in extracting the fundamental currentcomponent for each phase of the three-phase output by way of [Eqn. 2]and [Eqn. 3], no total Fast Fourier Transform (FFT) is employed in thecalculations, such that the computational burden imposed on controller32 is minimized.

In addition to analyzing the three-phase current in order to extract thefundamental current component, technique 50 also analyzes thethree-phase current in order to extract the third harmonic component foreach phase of the three-phase output, as indicated at STEP 58. The thirdharmonic component for each phase of the three-phase output is extractedaccording to:

$\begin{matrix}{{\frac{1}{2\pi}{\int_{0}^{2\pi}{\left( {{I_{1}\cos\;\theta} + {I_{3}\cos\; 3\theta}} \right)*\cos\; 3\;\theta*{\mathbb{d}\theta}}}} = \frac{I_{3}}{2}} & \left\lbrack {{Eqn}.\mspace{14mu} 4} \right\rbrack \\{{{\frac{1}{2\pi}{\int_{0}^{2\pi}{\left( {{I_{1}\cos\;\theta} + {I_{3}\cos\; 3\theta}} \right)*\sin\; 3{\mathbb{d}\theta}*{\mathbb{d}\theta}}}} = 0},} & \left\lbrack {{Eqn}.\mspace{14mu} 5} \right\rbrack\end{matrix}$where I₁ is the amplitude of the fundamental current component, I₃ isthe amplitude of the third harmonic component, and θ is the speed atwhich the load is commanded to run. In extracting the third harmoniccomponent for each phase of the three-phase output, both [Eqn. 4] and[Eqn. 5] are performed in order to run a self-check, with the output of[Eqn. 5] being zero when the calculations are performed properly. Again,the third harmonic component is calculated without using a total FFT soas to minimize the computational burden imposed on controller 32.

Upon extraction of the fundamental and third harmonic components,technique 50 then continues at STEP 60, where a determination is made asto whether the fundamental component and/or third harmonic componentextracted from each phase of the three-phase output exceeds apre-determined threshold that is set for the fundamental and thirdharmonic components. The pre-determined thresholds for the fundamentalcomponent and the third harmonic component will vary depending on thenormal fundamental component and third harmonic component levels (i.e.,fundamental and third harmonic can be 60/180 Hz, 50/150 Hz, etc.).

If it is determined at STEP 60 that the extracted fundamental componentand third harmonic component of any phase of the three-phase currentoutput exceed the pre-determined thresholds, indicated at 62, thentechnique continues at STEP 64 where a HRG fault is declared as beingpresent in power distribution system 10 and an alarm is activated, suchas some sort of audible or visual alarm. In declaring that an HRG faultis present in power distribution system 10, the location of the fault ona particular phase is also identified. That is, by determining on whichphase of the three-phase current output the extracted fundamentalcomponent and third harmonic component exceed the current thresholds,the location of the fault on a particular phase is easily achieved.Appropriate steps can then be implemented to protect components in powerdistribution system 10 from damage, such as damage that might occur tothe transformer windings insulation.

Conversely, if it is determined at STEP 60 that the extractedfundamental component and third harmonic component of any phase of thethree-phase current output do not exceed the pre-determined thresholds,indicated at 66, then technique loops back to STEP 52, where additionalmeasuring and analyzing of the phase current from each of the threephases of the ASD output is performed in order to continue monitoringfor an HRG fault.

Referring now to FIGS. 3 and 4, HRG fault characteristics and faultharmonics characteristics in power distribution system 10 (FIG. 1) areillustrated, before and after an HRG fault occurs. In the example, shownin FIGS. 3 and 4, the power distribution system is shown as operatingnormally until a time t=1.4 seconds, at which time a ground fault occursin one phase (e.g., Phase C).

With respect to FIG. 3, the upper window 70 therein is illustrative ofthe motor speed 72 and DC link voltage 74, while the middle window 76 isillustrative of the ASD three-phase output currents 78, 80, 82 and thelower window 84 is illustrative of the ground current 86. As can be seenin FIG. 3, the motor speed 72 remains in a steady state before and afteroccurrence of the ground fault 86 and the DC link voltage 74 remainsalmost undisturbed. FIG. 3 also shows that the output current on thefaulted phase, such as phase current 78, changes in amplitude afteroccurrence of the ground fault 86. While this change in amplitude ofphase current 78 can be tolerated in the system, the current contributesto the overall fault condition in the power distribution system.

With respect to FIG. 4, the upper window 88 therein is illustrative ofthe current components for each phase of the three-phase current output,while the lower window 90 is illustrative of the ground fault current.As can be seen in upper window 88 of FIG. 4, the amplitude of thefundamental component on the faulted phase, identified as 92, is greaterthan the amplitude of the fundamental component on the non-faultedphases (e.g., 130 amps versus 110 amps). Additionally, the amplitude ofthe third harmonic component on the faulted phase, identified as 94, isgreater than the amplitude of the third harmonic component on thenon-faulted phases. More specifically, the amplitude of the thirdharmonic component on the non-faulted phases will be zero, while theamplitude of the third harmonic component 94 on the faulted phase may be10 amps, for example. As shown in the lower window 90, the ground faultcurrent 96.

Thus, as illustrated in FIG. 4, not only can the presence of an HRGfault in the power distribution system be detected based on an analysisof the fundamental and third harmonic components of each phase of thethree-phase current output, but the phase and location on/at which theHRG fault occurs can be readily identified.

Beneficially, embodiments of the invention thus provide a system andmethod of ground fault detection and protection in power distributionsystems, including those incorporating an ASD and having an HRG systemconfiguration. The detection methods implement already availablethree-phase output current measurements and analyze those currentmeasurements in a computationally efficient manner to identify andlocate HRG faults in the system. Embodiments of the invention not onlyidentify a ground fault condition in a power distribution system, butalso determine exactly which phase out of three motor outputs that aground fault occurs on.

A technical contribution for the disclosed method and apparatus is thatit provides for a computer implemented technique for detecting groundfaults in a power distribution system, including systems having an ASDwith a HRG configuration. The technique extracts fundamental and thirdharmonic current components from each phase of a measured three-phasecurrent to detect HRG faults in an ASD, with the technique enablingidentification of which phase the ground fault is present on.

According to one embodiment of the present invention, a powerdistribution system includes a converter-inverter arrangement having aninput connectable to a three phase AC power source and a three phaseoutput connectable to an input terminal of a load, theconverter-inverter arrangement configured to control current flow andterminal voltages in the load. The power distribution system alsoincludes a fault detection and protection system connected to theconverter-inverter arrangement, with the fault detection and protectionsystem including a plurality of current sensors configured to measure acurrent on the three phase output of the converter-inverter arrangementand a controller configured to measure the three phase current on thethree phase output of the converter-inverter arrangement, extract afundamental current component for each phase of the three phase outputof the converter-inverter arrangement, extract a third harmoniccomponent for each phase of the three phase output of theconverter-inverter arrangement, compare the fundamental currentcomponent and the third harmonic component extracted from each phase toa first threshold and a second threshold, respectively, and detect aground fault on a phase of the three phase output based on thecomparisons of the fundamental current component and the third harmoniccomponent to the first and second thresholds.

According to another embodiment of the present invention, a method isprovided for detecting a high resistance ground fault in a powerdistribution system that includes an AC motor drive in series between anAC power source and an AC motor, with the AC motor drive configured tocondition a three phase output to the AC motor. The method includesmeasuring current on each of a first phase, a second phase, and a thirdphase of the three phase output to the AC motor, extracting afundamental current component for each of the first phase, second phase,and third phase, and extracting a third harmonic component for each ofthe first phase, second phase, and third phase. The method also includescomparing the fundamental current component and the third harmoniccomponent extracted from each of the first phase, second phase, andthird phase to a fundamental component threshold and a third harmonicthreshold, respectively and detecting a ground fault on any of the firstphase, second phase, and third phase based on the comparisons of thefundamental current component and the third harmonic component on eachphase to the fundamental and third harmonic thresholds. If a groundfault is detected, the method further includes identifying on which ofthe first phase, second phase, or third phase the ground fault ispresent.

According to yet another embodiment of the present invention, a systemfor detecting a ground fault in a high resistance ground (HRG) powerdistribution system includes a plurality of current sensors to measurecurrent on a three phase output of an inverter in the HRG powerdistribution system and a controller configured to measure the threephase current on the three phase output of the inverter, extract afundamental current component for each phase of the three phase output,extract a third harmonic component for each phase of the three phaseoutput, compare the fundamental current component and the third harmoniccomponent extracted from each phase to a fundamental component thresholdand a third harmonic component threshold, respectively, and detect aground fault on a phase of the three phase output based on thecomparisons of the fundamental current component and the third harmoniccomponent on each phase to the fundamental component and third harmoniccomponent thresholds.

The present invention has been described in terms of the preferredembodiment, and it is recognized that equivalents, alternatives, andmodifications, aside from those expressly stated, are possible andwithin the scope of the appending claims.

What is claimed is:
 1. A power distribution system comprising: aconverter-inverter arrangement having an input connectable to a threephase AC power source and a three phase output connectable to an inputterminal of a load, the converter-inverter arrangement configured tocontrol current flow and terminal voltages in the load; and a faultdetection and protection system connected to the converter-inverterarrangement, the fault detection and protection system comprising: aplurality of current sensors configured to measure a current on thethree phase output of the converter-inverter arrangement; and acontroller configured to: measure the three phase current on the threephase output of the converter-inverter arrangement; extract afundamental current component for each phase of the three phase outputof the converter-inverter arrangement; extract a third harmoniccomponent for each phase of the three phase output of theconverter-inverter arrangement; compare the fundamental currentcomponent and the third harmonic component extracted from each phase toa first threshold and a second threshold, respectively; detect a groundfault on a phase of the three phase output based on the comparisons ofthe fundamental current component and the third harmonic component tothe first and second thresholds; and declare a ground fault when onephase of the three phase output has a fundamental current component thatexceeds the first threshold and a third harmonic component that exceedsthe second threshold.
 2. The power distribution system of claim 1wherein the controller is configured to identify which phase of thethree phase output the ground fault is present on.
 3. The powerdistribution system of claim 1 wherein the controller is configured togenerate an alarm when a ground fault is declared.
 4. The powerdistribution system of claim 3 wherein the controller is configured todefine the phase current on each phase of the three phase output as:i=I ₁*cos θ+I ₃*cos 3θ, where I₁ is the amplitude of the fundamentalcurrent component, I₃ is the amplitude of the third harmonic component,and θ is the speed at which the load is commanded to run.
 5. The powerdistribution system of claim 4 wherein the controller is configured toextract the fundamental current component without using a total fastfourier transform (FFT), the fundamental current component beingextracted according to:${\frac{1}{2\pi}{\int_{0}^{2\pi}{\left( {{I_{1}\cos\;\theta} + {I_{3}\cos\; 3\theta}} \right)*\cos\;\theta*{\mathbb{d}\theta}}}} = \frac{I_{1}}{2}$${{\frac{1}{2\pi}{\int_{0}^{2\pi}{\left( {{I_{1}\cos\;\theta} + {I_{3}\cos\; 3{\mathbb{d}\theta}}} \right)*\sin\;\theta*{\mathbb{d}\theta}}}} = 0},$where I₁ is the amplitude of the fundamental current component, I₃ isthe amplitude of the third harmonic component, and θ is the speed atwhich the load is commanded to run.
 6. The power distribution system ofclaim 4 wherein the wherein the controller is configured to extract thethird harmonic component according to:${\frac{1}{2\pi}{\int_{0}^{2\pi}{\left( {{I_{1}\cos\;\theta} + {I_{3}\cos\; 3{\mathbb{d}\theta}}} \right)*\cos\; 3\theta*{\mathbb{d}\theta}}}} = \frac{I_{3}}{2}$${{\frac{1}{2\pi}{\int_{0}^{2\pi}{\left( {{I_{1}\cos\;\theta} + {I_{3}\cos\; 3\theta}} \right)*\sin\; 3\theta*{\mathbb{d}\theta}}}} = 0},$where I₁ is the amplitude of the fundamental current component, I₃ isthe amplitude of the third harmonic component, and θ is the speed atwhich the load is commanded to run.
 7. The power distribution system ofclaim 1 wherein the third harmonic component is zero on phases wherethere is no ground fault.
 8. The power distribution system of claim 1further comprising a resistor positioned between a neutral point of thethree phase AC power source and earth ground, such that the powerdistribution system comprises a high resistance ground (HRG) system. 9.The power distribution system of claim 8 wherein the converter-inverterarrangement comprises an adjustable speed drive (ASD), and wherein thecontroller is further configured to filter out switching frequencycomponents from the measured three phase current generated by switchingof a plurality of switches in the ASD.
 10. A method for detecting a highresistance ground fault in a power distribution system that includes anAC motor drive in series between an AC power source and an AC motor,with the AC motor drive configured to condition a three phase output tothe AC motor, wherein the method comprises: measuring current on each ofa first phase, a second phase, and a third phase of the three phaseoutput to the AC motor; extracting a fundamental current component foreach of the first phase, second phase, and third phase; extracting athird harmonic component for each of the first phase, second phase, andthird phase; comparing the fundamental current component and the thirdharmonic component extracted from each of the first phase, second phase,and third phase to a fundamental component threshold and a thirdharmonic threshold, respectively; detecting a ground fault on any of thefirst phase, second phase, and third phase based on the comparisons ofthe fundamental current component and the third harmonic component oneach phase to the fundamental and third harmonic thresholds; and if aground fault is detected: identifying on which of the first phase,second phase, or third phase the ground fault is present; and declaringa ground fault when a phase of the three phase output has a fundamentalcurrent component that exceeds the fundamental component threshold and athird harmonic component that exceeds the third harmonic threshold. 11.The method of claim 10 further comprising defining the phase current oneach of the first phase, second phase, and third phase as:i=I ₁*cos θ+I ₃*cos 3θ, where I₁ is the amplitude of the fundamentalcurrent component, I₃ is the amplitude of the third harmonic component,and θ is the speed at which the load is commanded to run.
 12. The methodof claim 10 wherein extracting the fundamental current componentcomprises extracting the fundamental current component according to:${\frac{1}{2\pi}{\int_{0}^{2\pi}{\left( {{I_{1}\cos\;\theta} + {I_{3}\cos\; 3\theta}} \right)*\cos\;\theta*{\mathbb{d}\theta}}}} = \frac{I_{1}}{2}$${{\frac{1}{2\pi}{\int_{0}^{2\pi}{\left( {{I_{1}\cos\;\theta} + {I_{3}\cos\; 3\theta}} \right)*\sin\;\theta*d\;\theta}}} = 0},$where I₁ is the amplitude of the fundamental current component, I₃ isthe amplitude of the third harmonic component, and θ is the speed atwhich the load is commanded to run.
 13. The method of claim 10 whereinextracting the third harmonic component comprises extracting the thirdharmonic component according to:${\frac{1}{2\pi}{\int_{0}^{2\pi}{\left( {{I_{1}\cos\;\theta} + {I_{3}\cos\; 3\theta}} \right)*\cos\; 3\theta*{\mathbb{d}\theta}}}} = \frac{I_{3}}{2}$${{\frac{1}{2\pi}{\int_{0}^{2\pi}{\left( {{I_{1}\cos\;\theta} + {I_{3}\cos\; 3\theta}} \right)*\sin\; 3\theta*{\mathbb{d}\theta}}}} = 0},$where I₁ is the amplitude of the fundamental current component, I₃ isthe amplitude of the third harmonic component, and θ is the speed atwhich the load is commanded to run.
 14. The method of claim 10 furthercomprising filtering out switching frequency components from the currentmeasured on each of the first phase, second phase, and third phase, soas to further isolate the fundamental current component and the thirdharmonic component.
 15. A system for detecting a ground fault in a highresistance ground (HRG) power distribution system, the systemcomprising: a plurality of current sensors to measure current on a threephase output of an inverter in the HRG power distribution system; and acontroller configured to: measure the three phase current on the threephase output of the inverter; extract a fundamental current componentfor each phase of the three phase output; extract a third harmoniccomponent for each phase of the three phase output; compare thefundamental current component and the third harmonic component extractedfrom each phase to a fundamental component threshold and a thirdharmonic component threshold, respectively; detect a ground fault on aphase of the three phase output based on the comparisons of thefundamental current component and the third harmonic component on eachphase to the fundamental component and third harmonic componentthresholds; and declare a ground fault when one phase of the three phaseoutput has a fundamental current component that exceeds the fundamentalcomponent threshold and a third harmonic component that exceeds thethird harmonic component threshold.
 16. The system of claim 15 whereinthe controller is configured to identify which phase of the three phaseoutput the ground fault is present on based on the extraction of thefundamental current component and the third harmonic component extractedfrom each phase.
 17. The system of claim 15 wherein the third harmoniccomponent is zero on phases where there is no ground fault.